Laboratory tests found that Diet Coke contained 139mg of caffeine per litre, compared to 109mg in regular Coca-Cola

Diet cola drinks contain higher levels of caffeine than regular versions, according to new research.

Caffeine is linked to hyperactivity in children, however manufacturers such as Coca-Cola, Pepsi and energy drinks companies are not required to label levels in their products.

As a result, families do not realise that apparently healthier diet drinks may contain more caffeine, which can effect behaviour.

Laboratory tests found that Diet Coke contained 139mg of caffeine per litre, compared to 109mg in regular Coca-Cola.

This equates to 46mg of caffeine in a normal-sized 330ml can of Diet Coke compared to 36mg in a can of regular Coca Cola, according to the research commissioned by the Sunday Times.

Diet Pepsi also had more caffeine, with 135mg per litre compared to 122mg in its regular product.

The tests showed that caffeine levels also fluctuated in the own-brand cola drinks produced by supermarkets. The Morrisons' version contained the most caffeine, with 124mg per litre, while Tesco's Diet Cola contained the least at 95mg per litre.

In December EU rules will come into force that require companies to put ‘high caffeine content’ warnings on products containing more than 150mg of caffeine per litre.

However, this regime will not cover many cola drinks which fall just below this threshold.

Nutritionists and health experts have warned that high-caffeine drinks, often consumed by children, could pose a risk to health and said the test results showed that more needed to be done to safeguard children.

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Research last year by the European Food Safety Authority found that more than two-thirds of British 10 to 18-year-olds had drunk high caffeine drinks.

Energy drinks such as Monster are a particular cause for concern, with some teenagers regularly taking cans in to school in the belief it will make them more alert in lessons or perform better in sports.

John Vincent, a government adviser on nutrition and co-founder of the Leon chain of restaurants, has called for better labelling on products and for all highly caffeinated drinks to be banned in schools.

Research last year by the European Food Safety Authority found that more than two-thirds of British 10 to 18-year-olds had drunk high caffeine drinks with Monster Energy (right) a particular cause for concern